The following results were obtained for the development of the upper lip and the pathogenesis of cleft lip based on studies using whole embryo cultures of A/WySnj mouse from D. 11. 5 (T. S.=19) for 24 hours.
1. Complete cleft lip occurred in embryos with underdeveloped facial prominences and a long distance between prominences.
2. Complete cleft lip occurred in embryos with a deep nasal groove due to tearing off of the nasal groove.
3. Incomplete cleft lip occurred in embryos with a normal nasal groove and a short distance between facial prominences.
4. Normal primary palates resulted in the development of a normal upper lip or an incomplete cleft lip with a persistent labial groove between the medial nasal and the maxillary prominences.
5. These results suggested that the pathogenesis of cleft lip is due to insufficient development of the facial prominences. Early onset of underdevelopment resulted in a severe cleft lip, while later onset resulted in a mild cleft lip.
